a) Field of the Invention
This invention relates to the use of a sulphur-containing pozzolan in blended hydraulic cements; more especially the invention relates to a blended hydraulic cement containing the pozzolan and to a pozzolan blend for hydraulic cements.
b) Description of Prior Art
A pozzolan is a siliceous or siliceous and aluminous material which possesses little or no cementitious value, but which, in finely divided form, will react with calcium hydroxide, in the presence of moisture to form a composition with cementitious properties.
Fly ash is a man-made pozzolan resulting from the combustion of coal and typically contains silicon dioxide, aluminium oxide, ferric oxide and calcium oxide with a small amount of sulphur typically not more than 5%, by weight, determined as sulphur trioxide.
Portland cement or clinker is a mixture of compounds, the main compounds being tricalcium silicate, dicalcium silicate, tricalcium aluminate and tetracalcium aluminoferrite. The binding or cementitious quality of Portland cement results from the hydration reaction between the cement and water. In particular, the two calcium silicates form calcium hydroxide and a calcium silicate hydrate which is the most important cementing component.
The rate of the hydration reaction can be regulated by the addition of a retarder, for example, gypsum; and also by fine grinding of the cement, but grinding of cement components is costly in energy and time consuming, addition of gypsum also increases cost.
Pozzolans are used with Portland cement in concrete production. Concrete is a mixture of the cement, water and mineral aggregate. The pozzolan may be added separately from the Portland cement at the concrete mix plant, or it may be premixed with Portland cement to form a blended cement.